Because PD-1 therapies only benefit approximately 1 in 5 patients with NSCLC, there is a great need to improve immunotherapy for this disease. A distinct form of immunotherapy, IL-2, can yield complete responses in melanoma and renal cell carcinoma. Use of IL-2 is limited due to common life-threatening toxicity. An alternative to IL-2 is IL-15/IL-15R? complexes. ALT-803 is a clinical grade IL-15/IL-15R? complex and powerful super-agonist for IL-15 responsive CD8+ lymphocytes and natural killer (NK) cells with dramatically improved safety compared with IL-2. Preclinical studies combining ALT-803 with anti-PD-1 mAb demonstrate remarkable anti-tumor efficacy associated with expansion of both CD8+ T cells and NK cells, and the production of inflammatory cytokines such as IL-6 and IFN?. We have moved this therapy into the clinic with an investigator- initiated, first-in-human, phase Ib/II trial testing the clinical hypothesis that adding IL-15/IL-15R? complexes (ALT- 803) to anti-PD1 mAb (nivolumab) is safe and efficacious in NSCLC (NCT02523469). To date 11 patients have been treated with three highly active dose levels (6, 10, 15 mcg/kg SC, 15 being the highest planned dose) with zero observed dose limiting toxicities (DLTs) and excellent activation of CD8+ T cells and NK cells and increases in serum IL-6 and IFN?. The goal of this application is to determine the mechanistic basis and predictors of response for this promising combinatorial approach using our unique bank of trial-derived samples and a powerful preclinical lung tumor model. We hypothesize that the addition of IL-15/IL-15R? complexes to anti-PD- 1 mAb augments anti-tumor efficacy through enhanced expansion and functional activation of tumor-reactive CD8+ lymphocytes and NK cells and that the induction of inflammatory cytokines such as IL-6 and IFN? will be associated with anti-tumor efficacy. We further hypothesize enhanced anti-tumor efficacy will depend on effector lymphocytes (CD8+ and NK but not CD4+) and correlate with cytokines (IL-6 and IFN?). Specific Aim 1: Using state-of-art mass cytometry we will perform the high-dimension characterization of both NK and CD8+ T cell compartments from longitudinal PBMC samples of up to 116 patients. We will perform TCR sequencing on expanded T cells to determine if they originate from tumor. We will also perform 65-plex serum cytokine/chemokine analysis of longitudinally acquired samples to identify novel serum biomarkers. We will associate changes in lymphocyte and serum cytokines/chemokines parameters with clinical response. Specific Aim 2: We will use our Lewis lung carcinoma (LLC) mouse model to inform our future clinical efforts with the combination of anti-PD-1 mAb and IL-15/sIL-15R? complexes. First, we will identify cellular and molecular markers that correlate with treatment outcome using longitudinal analysis of responding and non-responding mice. Second we will validate critical pathways using antibody-mediated depletion of lymphocyte subsets or cytokines. Finally, we will evaluate the effects of alternate dosing and schedule on key effector populations, and also integrate anti-CTLA-4 mAb therapy into our treatment.